The present invention relates to an automatic gain control (AGC) loop circuit, and more particularly to an automatic gain control loop circuit that controls two-channel input signals to be the same in level. The AGC loop circuit is employed in the color signal reproducing system of a videotape recorder (VTR) or the video signal processing system of a TV receiver.
In a domestic-use VTR, which performs recording/reproduction of video signals, the brightness signal component of a video signal is converted into a low-frequency carrier FM signal, while the color signal component thereof is converted into a down-converted color signal which is lower in frequency than the low-frequency carrier FM signal. The low-frequency carrier FM signal and the down-converted color signal are synthesized together, and the signal obtained thereby is magnetically recorded on videotape.
In a helical scan type VTR, videotape is wrapped around a rotatable cylinder and is made to travel in this state. A signal is recorded alternately in the two adjacent tracks of the videotape by means of two video heads that are mounted on the circumference of the rotatable cylinder.
At the time of reproduction, the two-channel signals are reproduced by the two video heads and alternately selected, thereby providing a continuous signal.
If the two video heads differ in performance or operation, signals having different amplitudes are alternately output. Usually, the signals with different amplitudes are switched from one to the other at the intervals of one scanning period. The output of such signals adversely affects the video signals, particularly the color component, resulting in so-called flickering.
In a conventional VTR, an automatic color amplification control (ACC) loop having an AGC function is incorporated in the color signal reproduction system. By the color signal amplification control of this loop, the color signal components of the signals reproduced by the two video heads are controlled to be the same in level, thereby suppressing the flickering.
FIG. 1 shows a signal reproduction system incorporated in a VTR.
Referring to FIG. 1, signals reproduced by two video heads (namely, "A"-head 41 and "B"-head 42) are amplified by play-back pre-amplifier circuits 43 and 44, respectively. Then, the signals are alternately selected by a switch circuit 45, thereby obtaining a continuous signal. The operation of switch circuit 45 is controlled by the control signal for controlling the operation of video heads 41 and 42.
A signal output from the switch circuit 45 is supplied to a color signal reproduction system. In this system, the signal is first input to LPF (low-pass filter) 46, so as to remove the brightness signal component (i.e., a low-frequency carrier wave FM signal) therefrom. A reproduced low-frequency band color signal, thus obtained, is supplied to an ACC loop circuit 10.
Conventionally, the ACC loop circuit 10 employed in the signal reproduction system shown in FIG. 1 has such a circuit configuration as is shown in FIG. 5, for example.
In the ACC loop circuit shown in FIG. 5, the two-channel signals alternately selected by the switch circuit 45 are first amplified by a variable-gain amplifier circuit 51 and then supplied to a frequency converter circuit 52. By this converter circuit 52, the signals are mixed with a 4.2 MHz local signal, so as to produce a 3.58 MHz-band carrier color signal. The carrier color signal is supplied to a next stage circuit (not shown) and a level detector circuit 53. The level detector circuit 53 detects the level of a color burst signal, and outputs first and second signals I1 and I2. The second signal I2 has a level substantially proportionate to a level of the first signal I1.
In this case, a first switch 54 is controlled such that the output level of the level detector circuit 53 is stored in capacitor C1 or C2 (which serves as a low-pass filter) during the channel period of the channel corresponding to one of the video heads 41 and 42. In addition, a second switch 55 is controlled such that the voltage kept in capacitor C1 or capacitor C2 is selected in accordance with the channel period and such that the selected voltage is fed back to the variable-gain amplifier circuit 51. The operation of switches 54 and 55 are controlled by the control signal for controlling the operation of video heads 41 and 42 (FIG. 1). According to the operation of the switches 54 and 55, the first and second signals I1 and I2 are fed back to the variable-gain amplifier circuit 51 as a gain control signals (negative feedback).
Since the two capacitors C1 and C2 hold the voltages corresponding to the two-channel color signal levels, no level difference between the color signals of the two channels (or the heads) can be obtained, thus suppressing the flickering. The flicker suppression system is referred to as a channel ACC system.
It should be noted, however, that the levels of the color signals do vary even in the channel periods though the variations themselves are slight. To cope with the level variations, each of the capacitors C1 and C2 serving as filters (at least two capacitors are required) has to be accurately controlled to have optimal capacitance. This results in an increase in the cost for the circuit components.
Moreover, since the two capacitors C1 and C2 serving as filters must have a high response characteristic, their capacitance is set to be relatively small. This being so, current leakage is likely to occur, and the voltages held in the capacitors become lower. Since a level difference is inevitably brought about when the reproduction signals corresponding to the video heads 41 and 42 switch from one to the other, the flickering cannot be suppressed completely.
As described above, the conventional automatic gain control loop circuit (which performs automatic gain control in such a way that two-channel signals alternately input are controlled to be the same in amplitude) requires at least two capacitors which serve as filters and which have such a small-valued capacitance as to appropriately respond to two-channel signals. When the two-channel signals switch from one to the other, a level difference is produced between the voltages held by the two capacitors, resulting in imperfect suppression of the flickering.